In geometry class, the girl to boy ratio is $3$ to $7$. If there are $21$ boys, how many girls are there?
Explanation: A ratio of $3$ girls to $7$ boys means that there is a group of $3$ girls for every group of $7$ boys. If there are $21$ boys, then there are $3$ groups of $7$ boys each. Because we know the ratio is $3:7$ , there must be $3$ groups of $3$ girls each. There is a total of $9$ girls in geometry class.